1. Field of the Invention
The present invention relates to a facsimile apparatus, printer or similar image forming apparatus and more particularly to an image forming apparatus of the type including a photoconductive element and an intermediate image transfer body having an elastic layer.
2. Description of the Background Art
A color image forming apparatus of the type including an intermediate image transfer body is conventional and forms a full-color image on a sheet or similar recording medium by the following procedure. A latent image is electrostatically formed on a photoconductive drum or similar image carrier and then developed by toner to become a toner image. The toner image is transferred to the intermediate image transfer body (primary image transfer. Such toner images of different colors are sequentially transferred to the intermediate image transfer body one above the other, completing a full-color image. Subsequently, the full-color image transferred from the intermediate transfer body to a sheet or recording medium (secondary image transfer).
A tandem, color image forming apparatus is a specific form of the color image forming apparatus of the type described and includes a plurality of photoconductive drums arranged side by side. In the tandem, image forming apparatus, an exclusive developing unit is assigned to each drum for forming a toner image on the drum in a particular color. The resulting toner images of different colors are sequentially transferred from the consecutive drums to an intermediate image transfer body one above the other, completing a full-color image. The intermediate transfer body is often implemented as an endless belt in order to reduce the size and cost of the apparatus. More specifically, a belt is advantageous over a drum, which is another specific form of the intermediate image transfer body, because it promotes free layout in the design aspect and saves at least a space corresponding to the volume of the drum.
In any case, the color image forming system using the intermediate image transfer body allows toner images of different colors to be brought into accurate register with each other, compared to a system of the type directly transferring toner images of different colors from a photoconductive drum to a sheet. Further, the system with the intermediate image transfer body effectively copes with defective image transfer and other problems ascribable to a difference in the property of a sheet.
For the secondary image transfer from the intermediate image transfer body to a sheet, use is made of, e.g., a bias roller positioned beneath the photoconductive drum. However, in a configuration that causes the bias roller to press the intermediate image transfer body, intense pressure locally acts at the secondary image transfer position and is apt to cause the center portion of, e.g., a character to be lost. Let this defect be referred to as the omission of the center of a character hereinafter.
Further, for the transfer of full-color images, various kinds of sheets including thick sheets, thin sheets and sheets of Japanese paper are often used. On the other hand, the conventional intermediate image transfer body is formed of fluorocarbon resins, polycarbonate resin, polyimide resin or similar resin and therefore too hard to deform complementarily to a toner layer. Consequently, the intermediate image transfer body is apt to compress a toner layer and bring about the omission of the center of a character. Particularly, when a full-color image is to be formed on a sheet having a rough surface, e.g., a Japanese paper sheet or a sheet intentionally formed with irregularity, a clearance is apt to appear between the sheet and toner and render a halftone portion or a solid portion irregular. Should image transfer pressure be intensified in order to obviate the above clearance, the cohesion of toner would be promoted and would aggravate the omission of the center of a character while increasing the amount of toner to be left on the intermediate image transfer body.
A cleaning device for cleaning the intermediate image transfer body includes a cleaning blade selectively movable into or out of contact with the intermediate image transfer body. When the operation of the image forming apparatus ends, the cleaning blade is released from the intermediate image transfer body and elastically restores its original position. This sometimes brings about a problem that the position where the cleaning blade contacts the intermediate image transfer body is slightly shifted, causing toner previously gathered by the cleaning blade to remain on the intermediate image transfer body in the form of a stripe. Such a stripe appears in the next toner image as a stripe-like smear.
Technologies relating to the present invention are disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 11-45011, 2000-155511 and 2000-310912.
It is an object of the present invention to provide an image forming apparatus capable of reducing the omission of the center of a character without exerting an excessive stress on toner existing on an intermediate transfer body at the time of image transfer.
It is another object of the present invention to provide an image forming apparatus capable of protecting an image from a stripe-like smear even when a cleaning blade is shifted from an expected position.
An image forming apparatus of the present invention includes a plurality of image forming means each including an image carrier, a charger for uniformly charging the surface of said image carrier, and a developing device for developing a latent image formed on the charged surface of the image carrier with toner to thereby produce a corresponding toner image. A primary image transferring device transfers such toner images from the image carriers to an intermediate image transfer body one above the other, thereby completing a composite toner image. A secondary image transferring device transfers the composite toner image from the intermediate image transfer body to a recording medium. The primary image transferring device includes the intermediate image transfer body includes at least an elastic layer, a cleaning unit for cleaning the intermediate image transfer body, and a coating member for coating a lubricant on the intermediate image transfer body.